When a user personally listens to reproduced music and voice, he/she often uses headphones such as earphones and stereo phones. Frequency characteristics of the sound output from headphones differ for different products, thereby the sound output from the headphones do not always have a user desired frequency characteristic.
Therefore, there are demands for the sound output from the headphones to have a user desired frequency characteristic.
Conventionally, it is difficult to appropriately correct the frequency characteristic of the earphone at when the user uses the earphone, because there is provided no means to objectively measure the correct frequency characteristic. On the other hand, when an equalizer is used for manual adjustment of the frequency characteristic, a user needs to subjectively adjust the equalizer while listening to musical sound. In this case, the user often repeats the adjustment by trial and error because the subjective adjustment is influenced by, for example, a sound source and the user's mood. Thus, it is difficult to properly correct the sound from the earphone. Furthermore, as a reproducible measurement technique, there is known a technique using a jig of a special type, but this requires the jig for every measurement.